​In an experimental design that investigates the effects of weight loss on heart rate in middle-aged men, the dependent variable would be

A) ​stress.
B) ​heart rate.
C) ​age.
D) ​gender.

The dependent variable is the variable that is being measured or tested in the study. In this case, the dependent variable is the heart rate of middle-aged men, as the study is investigating the effects of weight loss on heart rate. Stress, age, and gender are all potential independent variables, meaning they can be manipulated or controlled in the study to see how they affect heart rate. But the dependent variable is what we are looking to measure and observe.